Almond was walking to his car after a performance Monday night when an assailant used a stun gun on him, causing him to drop the instrument.

The attacker then took the violin and fled to a waiting vehicle, which was being driven by a second suspect, according to Milwaukee police. That vehicle was described as a late 1980s or early 1990s, maroon or burgundy, Chrysler or Dodge minivan.

"It appears at this time that the violin was the primary target of this assault and robbery," police said in a statement.

"It is important to note that this violin is valuable to a very small number of people in the world and is not something easily sold for what it is worth."

According to the symphony, Almond plays on a violin made by Antonio Stradivari from 1715, known as the "ex-Lipinski."

CNN affiliate WITI reports that it's worth an estimated $6 million.

A police spokesman declined to comment on the instrument's exact cost, instead putting it in the "millions."

The Milwaukee police are working with the FBI's Art Crime Team, which specializes in high-end art thefts. The FBI team, in turn, works with Interpol to connect art dealers able to help locate stolen items throughout the world.

Antonio Stradivari was born in 1644. He set up shop in Cremona, Italy, where he made violins, harps, guitars, violas and cellos.

Before his death in 1737, Stradivari is thought to have made more than 1,100 instruments -- of which some 650 remain today -- according to the Smithsonian Institution.